1. Field of the Invention
The present invention relates to a wiper control apparatus using a semiconductor switching element such as a MOSFET.
2. Description of the Prior Art
One of previously known wiper control apparatus is disclosed in JP-A-9-193748. In this wiper control apparatus, as seen from FIG. 13, when a combination switch 17 is set at an intermittent mode (INT), a driving circuit 20 turns on an interruption switch MOSFET 19 to start a motor 15 so that a wiper reciprocates once. Then, the terminals P and Q of a wiper switch 16 are connected to each other. Then, the driving circuit 20 turns off the interruption switch 19 to interrupt the power supply to the motor 15. When the terminals P and Q of the wiper switch 16 are connected, a battery-voltage side terminal K and a ground side terminal J are connected to each other through a contact terminal T1 and braking resistor 18 so that the motor 15 is braked. Thus, the wiper stops quickly.
However, the wiper control apparatus presents the following problems.
(1) When the wiper motor 15 generates an abnormal current as if it is locked, a large current continues to flow through a MOSFET 19 or resistor 18. Therefore, the resistor 18 or MOSFET 19 may be excessively heated and burned. In order to avoid the burning, a countermeasure for setting a large current rating for the resistor 18 and MOSFET 19 and providing a radiator is required. This upsizes the apparatus and makes it expensive.
(2) When the plus terminal and minus terminal of a battery 11 are erroneously connected inversely, a large current continues to flow through the parasitic diode of the MOSFET 19 and the resistor 18. Therefore, the resistor 18 and the MOSFET may be burned.
(3) For some time after the MOSFET 19 turns on, since the terminals P and Q of the wiper switch 16 remain connected, a tunneling current will flow through a passage of wiper switch 16xe2x86x92resistor 18xe2x86x92MOSFET 19 from the battery 11. This leads to unnecessary dissipation of heat and noise.
(4) Setting the intermittent time and the after-wash wiping operation time after the washer has been operated is carried out in an analog manner using the charging/discharging theory of a capacitor. This leads to a large change in these times.
In order to solve the problems (1) to (4) of the above wiper control apparatus, JP-A-2000-335374 proposed a wiper control apparatus which is compact, reliable and surely operable using a semiconductor switch and a digital controller.
In the proposed wiper control apparatus, also when the abnormality of the wiring downstream of the wiper motor 15 being short-circuited to a chassis (ground) occurs, a large current continues to flow through the resistor 18 so that the resistor may be excessively heated and burned.
Now it is assumed that short-circuiting of the wiring to the chassis (ground) has occurred between the ground side terminal J and the combination switch 17. In this case, a current flows via the battery-voltage side terminal K of ground side terminal J of the wiper motor 15 and the short-circuited point so that the wiper motor 15 rotates.
On the other hand, the contact P of the wiper switch 16 is periodically changed between the contact points Q and R. While the contact P is being in contact with the contact Q (about 0.1 sec), a large current flows via the contact P, resistor 18, combination switch 17 and short-circuited point. This also occurs between the combination switch 17 and the switch device and between the combination switch 17 and the resistor 18.
At this time, since the current is limited by the resistor 18 having a resistance of 2-4 xcexa9 (if the voltage of the battery 11 is 12 V, the current is 5 V at the maximum), the fuse will not be broken. As a result, the current continues to flow through the resistor 18 so that it may be excessively heated and burned. In order to avoid the burning, the countermeasure of providing a radiator with the current rating of the resistor 18 being increased is required. This increases the scale of the apparatus and the production cost thereof.
FIG. 14 shows timing charts of signals in an ignition switch (IG) of a wiper control apparatus 12, a motor AS switch 16, wiper motor 16 and brake resistor 18. As seen from FIG. 14, when abnormality of short-circuiting occurs downstream of the wiper motor 15, a current continues to flow through the wiper motor 15 so that the contact point of the motor AS switch 16 is periodically changed between the high side and low side. Correspondingly, the current continues to flow through the braking resistor 18.
An object of the present invention is to improve the wiper control apparatus proposed in JP-A-2000-335374 and to solve the problem when abnormality of the wiring downstream of a wiper motor being short-circuited occurs in the conventional wiper control apparatus to provide a wiper control apparatus which is compact, reliable and surely operable using a semiconductor switch and a digital controller.
In order to attain the above object, in accordance with the first aspect of the present invention, there is provided a wiper control apparatus comprising:
a combination switch for setting an operation mode of a wiper;
a wiper motor which is driven according to the operation mode set by the combination switch;
an autostop (AS) switch for producing a signal having a level varying according to a park position and other positions as the wiper motor rotates;
a first semiconductor switch element for on/off controlling energization of the wiper motor;
a second semiconductor switch element for turning on a closed circuit for braking the wiper motor by passing a reverse current through the wiper motor while energization for the wiper motor is off;
a controller for supplying a first ON/OFF control signal to the first semiconductor switch element and a second ON/OFF control signal to the second semiconductor switch element;
current detecting means for detecting a current flowing through the second semiconductor switch; and
current limiting means for limiting a current flowing through the second semiconductor switch element if the current detected by the current detecting means exceeds a prescribed threshold value.
In this configuration, since an abnormal current is limited when abnormality of short-circuiting occurs downstream of the wiper motor, the safety of the wiper control apparatus can be improved. Since heat generation can be reduced when the apparatus is in a normal operation and abnormality of short-circuiting occurs, the wiper control apparatus can be downsized.
Preferably, the current limiting means interrupts the second ON/OFF control signal the current limiting means interrupts the second ON/OFF control signal when the number of limiting the current reaches a prescribed number of times.
In this configuration, when the number of limiting the current reaches a prescribed number of times, the closed circuit for braking can be surely interrupted so that the safety of the wiper control apparatus can be improved.
Preferably, the first semiconductor switch element is an N-channel MOSFET, and the second semiconductor switch element is a P-channel MOSFET.
In this configuration, the wiper control apparatus can be downsized and manufactured at low cost.
Preferably, a Zener diode is connected to the semiconductor switch element.
In this configuration, inverted electromotive force when the first semiconductor switch is excessively heated and cut off and interrupted is dissipated. The first semiconductor switch can be protected when a battery is erroneously connected in a reverse direction. The second semiconductor switch element can be surely turned off in a manner of shifting the electric potential of its gate electrode.
Preferably, the first semiconductor switch element is located downstream or upstream of the wiper motor.
Where the first semiconductor switch element is arranged upstream, it is protected from malfunction such as short-circuiting so that the safety for the apparatus is improved.
Preferably, the wiper control apparatus according to any one of the first to fifth aspects can be employed together with a relay-type wiper control apparatus, and the controller controls the operation of both wiper control apparatus and relay-type control apparatus.
In this configuration, the controller includes the collected function of controlling both the semiconductor switch(s) and relay, and hence down-sized and made inexpensive. The control is correlated between the front wiper and rear wiper.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.